Spark plugs are used to cause a spark discharge for igniting air-fuel mixture in a combustion chamber. An example of a spark plug having a known structure includes an insulator having an axial hole that extends through the insulator along an axis; a metal shell that holds the insulator therein; a center electrode held in the axial hole; and a conductive sealing member that holds the center electrode in the axial hole (International Publication No. 2012/105255). In the structure disclosed in International Publication No. 2012/105255, the center electrode includes a flange portion that protrudes in a radial direction, and a head portion that projects from the flange portion toward a back side. This structure enables the center electrode to be held in the insulator. More specifically, the flange portion abuts against a step portion provided in the axial hole so that the center electrode does not move toward a front side. In addition, a sealing member is provided around the head portion and the flange portion so that the center electrode has sufficient shock resistance and the center electrode does not become loose when the center electrode receives the impact of combustion.
The spark plug is required to include electrodes having sufficient durability against repeated spark discharge. The durability can be effectively increased by reducing the capacitance between the metal shell and a conductor disposed in the insulator. Here, the conductor is the sealing member or the center electrode. The capacitance can be reduced by, for example, reducing the length of the head portion and reducing the height of the sealing member in the axial direction by an amount corresponding to the reduction in the length of the head portion. However, when the length of the head portion is reduced, the retaining force of the sealing member is also reduced. Accordingly, the shock resistance of the center electrode is reduced and the center electrode easily becomes loose. In light of the above-described circumstances, an advantage of the present invention is to reduce the capacitance and ensure that the center electrode has sufficient shock resistance at the same time.
The present invention achieves the above-described advantage, and is realized in the following forms.